A New Study of Bacterial Motion: Superconducting Quantum Interference Device Microscopy of Magnetotactic Bacteria

Abstract

The recently developed "microscope" based on a high-Tc dc SQUID (superconducting quantum interference device) is used to detect the magnetic fields produced by the motion of magnetotactic bacteria, which have permanent dipole moments. The bacteria, in growth medium at room temperature, can be brought to within 15 micron of a SQUID at liquid nitrogen temperature. Measurements are performed on both motile and nonmotile bacteria. In the nonmotile case, we obtain the power spectrum of the magnetic field noise produced by the rotational Brownian motion of the ensemble of bacteria. Furthermore, we measure the time-dependent field produced by the ensemble in response to an applied uniform magnetic field. In the motile case, we obtain the magnetic field power spectra produced by the swimming bacteria. Combined, these measurements determine the average rotational drag coefficient, magnetic moment, and the frequency and amplitude of the vibrational and rotational modes of the bacteria in a unified set of measurements. In addition, the microscope can easily resolve the motion of a single bacterium. This technique can be extended to any cell to which a magnetic tag can be attached.

Characterization and Control of Biological Microrobots

“…Further, L and d are the length and diameter of the MTB, respectively [14]. The flip-time can be determined experimentally, then the magnetic dipole moment can be calculated by solving (3).…”

“…Our MTB, i.e., Magnetospirillum magnetotacticum (MS-1), provides propulsion by rotating its helical flagella at ∼628.3 rad/s [14]. Alternating the direction of the field lines could perturb the desirable modes of the MTB, and hence decreases its velocity.…”

Magnetic susceptibility measurement of single iron/cobalt carbonyl microcrystal by atmospheric magnetophoresis

“…By this method, a magnetic momentum less than 10 −16 A m 2 can be detected by measuring the force of the pN order very close to the sample (∼ 5 nm) [43] with a spatial resolution less than 30 nm. The SPM utilizing micro-SQUID as a probe is called a scanning SQUID microscope, whose sensitivity makes it possible to measure the magnetic momentum of about 10 −17 A m 2 [44]. However, these two methods have disadvantages in measuring the induced magnetic momentum of a microparticle.…”

Optical Doppler shift with structured light

“…The capability to detect transverse velocities is of great interest in fields as diverse as biology microfluidics, microorganism motility [1], optical coherence tomography medical imaging [2], atmospheric and oceanic turbulence remote sensing [3], and fluid aerodynamics [4].…”

“…For example, for a typical value of motility [1] of a biological specimen of tens of micrometers per second, a linear spatial modulation of γ ∼ 1 μm −1 would yield a Doppler frequency shift of some tens of hertz. This scheme can also be used to measure fluid flow in live tissue, where the possibility of inducing tiny phase gradients that would not affect the in vivo system under study can be of great interest.…”